In operations of modification of the cylinders in a drying group in a paper machine, the drying cylinder has been removed from its place and carried aside for machining. At present, ordinary non-perforated cylinders in a drying group are converted to so-called UNO-VAC.RTM. cylinders by perforating the cylinder mantle. In prior art, the perforation had to be carried out at a separate machining point either at the paper mill or in a workshop specialized in the conditioning of cylinders.